This invention relates generally to a method and apparatus for speed control of steam turbines. More specifically, the invention relates to a method for overcoming performance degradation of a worn or defective pilot-valve assembly (a component of the control system) or other control system components, by monitoring the performance of the control system and employing one or more additional, digital controllers; as a result, improving the overall accuracy of the turbine speed-control system.
To govern the speed and power of a steam turbine, a valve (or more commonly, a set of valves) must be adjusted to vary the flow of steam through the turbine. Typically, such valves are regulated by a hydraulic steam-valve actuator which, in turn, is activated by a pilot valve modulated by an electromechanical actuator that receives its signal from a speed-control system.
Present-day speed control systems for steam turbines include a proportional-integral-differential (PID) controller that utilizes signals representing rotational speed. This speed controller then transmits an actuator-position set point to another PID controller that monitors steam-valve actuator position and whose output activates (indirectly) the steam-valve actuator to render its position equal to the actuator set point. In reality, the steam-valve actuator controller""s output is employed as a set point for an electromechanical actuator which modulates a pilot valve: hydraulic fluid is directed through the pilot valve to-and-from the steam-valve actuator to change its position. Pilot valves, steam-valve actuators, and other control system components can, however, suffer performance degradation due to manufacturing defects, wear, contaminated hydraulic fluid, and other ills, thereby impairing system performance. Consequently, a method of control is needed that compensates for these detrimental inefficiencies.
A purpose of this invention is to provide a method for controlling the rate of steam flow through a steam turbine by monitoring the position of a pilot valve along with the dynamics of the controlled system""s response, and using this information to compensate for the action of a faulty pilot-valve assembly and/or other control-system components that do not perform to standard.
To accomplish this purpose, control elements are added to the standard control system used to govern turbine speed; in particular, additional PID controllers are included. One of these units is dedicated to maintaining the position of the pilot valve at a set point obtained from a PID main controlled-parameter controller, such as a steam valve position, steam flow, or generator power. Therefore, the controller for pilot-valve position is cascaded with the controller for the main controlled-parameter.
A second controller is dedicated to controlled-system dynamics. Such dynamics can be detected by measuring any of several main controlled-parameters, such as steam-valve actuator position, output of a turbine load (a generator or a compressor), steam flow rate, and turbine pressure. For each of these options, a calculation function is required which takes the first time-derivative of the main controlled- parameter signal. The set point for this second controller is proportional to the difference (error) between the parameter""s set point and its actual value.
The resulting signal, inputted to the pilot valve""s electromechanical actuator, is proportional to a combination of the outputs from the two PID controllers described above. The nature of the combination (linear, for example) is subject to those requirements of the application and customer.